This invention relates to digital color image processing and, more particularly, to smoothing the digital color image.
With the advent of digital cameras, it is becoming more and more advantageous to capture images as colored digital images. Colored digital images are frequently stored in three color planes such as red, green, and blue, or cyan, magenta, and yellow. In image processing, these colored digital images luminance and chrominance color coordinates are quite useful because they express color in a similar fashion to the way the human visual system operates. As is well known, luminance, the black and white portion of an image, determines the sharpness of such image while the chrominance values determines its colorfulness.
As also is well known to those skilled in the art, generally three channels are used to describe a color. For example, if an image is recorded having red, green, and blue channels, this can be converted to one luminance channel and two chrominance channels such as Y, Cr, Cb. These luminance and two chrominance channels facilitate certain aspects of digital image processing. Color interpolation between pixels can reduce the noise level in the luminance channel, but not without the expense of increasing noise in the chrominance channel. If this noise if left untreated, artifacts can appear in an output image which often are characterized by being splotchy in appearance.
This splotchy color problem can be reduced by smoothing the chrominance channels. Because the chrominance content of the images is usually of low spatial frequency and the noise content is of high spatial frequency, the application of a low-pass blur filter has been found to reduce noise levels without significantly reducing image quality. However, the higher the noise level, the more aggressive the smoothing or blurring should be.
A problem with chrominance smoothing occurs at the edges in a digital image. If the blurring is aggressive (passing lower frequencies) and the colors near the edge are bold (have high chroma), the blurring will cause the colors to xe2x80x9cbleedxe2x80x9d across the edge. In this situation, the image around the edge is easily seen by a viewer to be unnatural.
It is an object of the present invention to provide an effective way of smoothing digital color images which minimizes the problem of color bleeding across image edges discussed above.
This object is achieved by a method for smoothing a digital color image having color pixels in which each colored pixel is expressed as one luminance and two chrominance color values, such method comprising the steps of:
a) computing parameters for at least two orientations for each pixel in the stored digital image;
b) using the computed parameters to form a classifier value for each such orientation and using such classifiers to determine the pixel classification based on such classifiers; and
c) smoothing pixel chroma values according to the determined pixel classification.
An advantage of the present invention is the recognition that chroma values can be corrected in accordance with the present invention to provide highly effective smoothing of digital color image.
Another advantage of the present invention is that it greatly reduced color bleeding across edges. The method can be readily implemented and assigns each pixel to one of five classes which are used in color smoothing. This invention readily lends itself to digital image processing.